The body of a small head may be worked up from one piece, in much the same manner as a copper kettlespout. In Fig. 213 an elevation of a small head is shown. To mark out the pattern the centre line, A D, is drawn the same length as the curve a d, the lengths of the intermediate parts also corresponding to those of the elevation. The line A H is made equal to half the circumference of the ventilator shaft-pipe, and C E cut off equal to one and a half times the length of c e on the elevation. The outline of the top part of the pattern comes out as a semi-ellipse; and this can, perhaps, be best marked out by what is known as the "trammel method." On a strip of hoop-iron or a wooden lath mark from the end a distance equal to the semi-axes, or diameters, of the ellipse; in this case C D and C E respectively; thus obtaining two points like Y and Z, as shown on the sketch of trammel in Fig. 213. Fix the trammel in several successive positions, always keeping the points Y and Z on the lines C D and E E respectively, and mark the position of X; thus points for the required ellipse will be obtained, and when joined up with an even curve, will give the boundary of the top portion of the pattern, as shown. A curve is now run from E to join on to the pipe portion of the pattern.
For the bell-mouth, a ring in this case will be best; its diameter, F G, being equal to f g in the elevation. The width of ring F E will have to be somewhat larger than the length of arc f e, to allow for draw. This width can be calculated, but it will be sufficiently accurate to make F E equal to about one and a quarter times f e. The bell-mouth can be fixed to the body by making a knocked-up joint.
An enlarged view, showing the method of fixing the bead, which is usually split-tubing, is also shown in Fig. 213.